This invention relates to pre-emphasis and equalization methods and circuitry for data communication. Pre-emphasis and equalization are used to improve signal transmission and reception along transmission links by boosting the amplitude of particular signal components.
Transmission links, such as circuit traces, integrated circuit interconnect lines or pins, backplanes, wires or cables, antennas, air, or other transmission media, may attenuate, delay, or otherwise change the characteristics of signals traveling through them. Signal attenuation may be frequency dependent, different frequency signal components being attenuated by different amounts. In general, high frequency signal components are attenuated more than lower frequency components. Phase delay or group delay caused by the transmission link may also be frequency dependent, causing different signal frequency components to arrive at the receiver with different delays.
In dielectric conductors such as those commonly used in backplanes, transmission losses are mainly due to skin effect and dielectric losses. Skin effect losses have a square root dependence on frequency, and cause non-linear phase delay distortion. Dielectric losses increase approximately linearly with frequency, and result in approximately linear phase delay and constant group delay.
In order to counter-act the attenuation of the signals as they travel through the transmission media, transmitter circuits often use pre-emphasis techniques to increase the amplitude of particular signal components before transmission. For transmission in integrated circuit lines in which attenuation typically increases with frequency, pre-emphasis may amplify higher frequency components more than lower frequency components, such that all signal components arrive at the receiver with approximately the same amplitude. In addition to, or instead of using pre-emphasis, receiver circuits often use equalization techniques to improve signal transmission. Equalization may consist in amplifying certain components of the received signal, while leaving other components unchanged. In a manner similar to pre-emphasis, equalization may be used to counter-act the frequency-dependent attenuation caused by signal transmission.
While pre-emphasis and equalization methods and circuits for adjusting signal amplitude are well known, these methods typically do not adjust for phase delay introduced in the signal during transmission, or phase delay caused by the transmission, pre-emphasis or equalization circuitry. Phase delay distortion from these and other sources may reduce the quality of the received signal, and may adversely affect signal transmission. This invention provides methods and circuits for phase-adjusted pre-emphasis and equalization.